nvme: return string as char *, not unsigned char *

The functions in drivers/nvme/host/constants.c returning human-readable
status and opcode strings currently use type "const unsigned char *".
Typically string constants use type "const char *",
so remove "unsigned" from the return types.
This is a purely cosmetic change to clarify that the functions
return text strings instead of an array of bytes, for example.

Signed-off-by: Caleb Sander <csander@purestorage.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>